1. Field of the Invention
This invention relates to coating compositions based on aqueous polyurethane dispersions and to the use thereof for the production of top coats or finishes on flexible substrates, in particular on PVC. The present coating agents comprise an aqueous polyurethane dispersion having certain physico-chemical properties.
2. Description of the Prior Art
The production of polyurethane coatings on PVC is known in principle. Thus, for example, U.S. Pat. Nos. 3,262,805; 4,017,493 and 4,045,600 describe processes for the coating of PVC using one- or two-component polyurethane systems dissolved in organic solvents.
The use of solvent-containing coating agents is accompanied by numerous problems, however, and for economical and, in particular, ecological reasons, it is necessary to restrict as far as possible or even to avoid completely the use of organic solvents. It is known from German Offenlegungsschrift No. 1,769,387 (British Pat. No. 1,191,260) to cement PVC with aqueous polyurethane dispersions. However, the aqueous polyurethane dispersions could not be used as a top coat or finish on PVC as it has not been possible hitherto to provide products having, on the one hand, good adhesion to PVC and, on the other hand, the high level of quality demanded in the coatings industry for top coats and finishes (high softening point; high mechanical strength; stability to hydrolysis and ageing; minimal swelling in plasticizers; blocking action against the migration of the plasticizers contained in the PVC; avoidance of discoloration at the gelation temperatures of PVC pastes; and pleasant dry feel).
The coating agents described in German Offenlegungsschrift No. 2,807,479 made it possible for the first time to provide aqueous polyurethane dispersions which almost met the above requirements and allowed the production of coatings on PVC, in particular on foamed PVC, without the use of solvents.
However, these coating agents still have some disadvantages which make them difficult to use:
The flex resistance of the final composite material is often inadequate and the adhesion to the PVC is unsatisfactory.
In particular, however, the fact that the coating agents are composed of mixtures of two polyurethane dispersions is a considerable disadvantage. Such mixtures are invariably expensive and complicated to produce and may give rise to errors. Moreover, the compatibility of the two systems must be guaranteed.
It has now been found that the disadvantages of the system according to German Offenlegungsschrift No. 2,807,479 may be overcome and that it is still possible, with uniform polyurethane dispersions, to produce coatings on PVC which have an exceptional level of quality if a dispersion of a certain composition is used. This is all the more surprising since it is quite impossible with the individual dispersion components according to German Offenlegungsschrift No. 2,807,470 to produce coatings or finishes on foamed or non-foamed PVC which meet the essential requirements as regards quality.